Production of dicarboxylic acid anhydrides by the catalytic oxidation of olefins



United States Patent assignor to Petro-Tex a corporation of Thisinvention relates to an improved process for the manufacture ofdicarboxylic acid anhydrides by catalytic oxidation of ethylenicallyunsaturated hydrocarbons and relates more particularly to an improvedprocess for producing monoethylenically unsaturated aliphaticdicarboxylic acid anhydrides such as maleic anhydride by reacting amixture of an oxygen-containing gas and an ethylenically unsaturatedhydrocarbon in vapor phase in the presence of a novel catalyst therefor.

Production of dicarboxylic acid anhydrides by vapor phase catalyticoxidation of hydrocarbons is well known. The principal method currentlyemployed for making maleic anhydride is by the catalytic oxidation ofbenzene in the presence of certain heavy metal oxide catalysts. Althoughcatalysts for the oxidation of unsaturated aliphatic hydrocarbons areknown, the yields of the desired maleic anhydride product over the knowncatalysts are not substantially high enough to make such processescommercially attractive as compared to maleic anhydride prepared frombenzene. More efiicient conversion of the hydrocarbon to maleicanhydride and longer life catalysts are desirable.

It is, accordingly, an object of this invention to provide an improvedprocess for obtaining high yields of dicarboxylic acid anhydrides byvapor phase oxidation of olefins. It is another object of this inventionto provide an improved process for the vapor phase oxidation ofmonoolefins, particularly butene-Z to maleic anhydride, in yieldsgreaterthan about 70 weight percent. It is a further object of thisinvention to provide a novel and improved catalyst useful in obtainingincreased yield of product by vapor phasecatalytic oxidation of olefinsto aliphatic dicarboxylic acid anhydrides, and a method for making thesame. Another object is to produce catalysts of long life, Other objectsand advantages of the invention will be apparent from the descriptionthereof which follows. 1

It has now been found that particular. vanadium-oxygen-phosphoruscomplex type catalysts containing a minor amount of a phosphorusstabilizer are effective in converting olefins to dicarboxylic acidanhydrides at yields as high as about 90 weight percent or more underthe reaction conditions specified hereinafter. This substantial increasein conversion over that obtained in the prior art has obvious economicadvantages. These catalysts are characterized by improved long life,particularly because of the presence of the phosphorus stabilizer.

The phosphorus stabilizer of the catalyst is an element or mixture ofelements selected from arsenic, chromium, the rare earth elements andmetals from Groups lb, IIb, IIIa, lIIb, IVa, IVb, Va, and the metals inthe fourth period of Group VIIIb. The preferred element's are copper,silver, zinc, cadmium, aluminum, gallium, indium, scandium, yttrium,lanthanum, germanium, tin, lead, titanium, zirconium, antimony, bismuth,arsenic, iron, cobalt, nickel, cerium, praseodymium, neodymium andchromium and mixtures thereof. These elements may These groups are basedon the Periodic Table astound in Smiths Introductory College Chemistry;3rd ed. 11950) by William F. Ehret (Appleton-Century-Crofts, Tue).

3,156,705 Patented Nov. 10, 1964 "ice be introduced either as theelement itself or through the use of compounds of the elements.

The novel catalysts of this invention contain vanadium, oxygen,phosphorus, and the phosphorus stabilizer chemically bonded together asa complex.

The atomic ratio of phosphorus to vanadium should be maintained betweenabout one to two atoms of phosphorus per atom of vanadium. Preferably,the ratioof atoms of phosphorus to atoms of vanadium is from about 1.1to 1.6 atoms of phosphorus per atom of vanadium. The phosphorusstabilizer should be present in a total amount from about 0.05 to about5.0 percent, and more preferably from about 0.2 to about 2.0 weightpercent of the element based on the total weight of vanadium, oxygen andphosphorus.

The catalysts are prepared by combining the vanadium with a. phosphoruscompound. A vanadium oxysalt may be used wherein the anion of the acidused to form the vanadium oxysalt is more volatile than the anionofphosphoric acid. That is, the anion used is an anion of an acid which ismore volatile than phosphoric acid. When the vanadium oxysalt iscombined with the phosphorus compound a vanadium-oxygen-phosphoruscomplex is formed. The vanadium oxysalt may be either added as such orformed in situ during the preparation of the vanadium-oxygen-phosphoruscomplex. The phosphorus stabilizer may be introduced into the catalystin a num: ber of ways. The method of introduction of this addedphosphorus stabilizer may be any method used which results in the addedstabilizer being intimately and chemically combined with thevanadium-oxygen-phosphorus complex. The added phosphorus stabilizer maybe added during the preparation of the vanadium-oxygen-phosphoruscomplex, or the complex may first be prepared and the phosphorusstabilizer added either before, at the same time, or after either thevanadium or phosphorus compound is added. The phosphorus stabilizer maybe added before, after, or at the same time as the carrier, if any, isadded. If the phosphorus stabilizer contains more than one compound,these compounds may be added either together or separately, eitherbefore, during or after the reaction of the vanadium compound with thephosphorus compound.

7 Catalyst complexes which are prepared by a solution method arevaluable catalysts for this invention. For

example, if vanadium oxychloride is used, the solvent or inorganic.

may be hydrochloric acid. The vanadium oxychloride solution may bereadily obtained by dissolving vanadium pentoxide in concentratedhydrochloric acid. .The phosphorus may then be introduced by adding aphosphorus compound such as phosphoric acid or P 0 to the vanae diumoxychloride to form the vanadium-oxygen-phosphorus complex dissolved inthe hydrochloric acid. The added phosphorus stabilizer is normallydissolved along with the vanadium pentoxide in hydrochloric acid, or if,for example, vanadium oxychloride is the starting material, it may bedissolved in a solution thereof prior to the addition of phosphoricacid. or P 0 The rate .of

formation of the complex may be increased with the use of heat.

The vanadium oxysalt used in the preparation of the catalyst maydesirably have as the salt forming anionany anion of an acid which ismore volatile than the anion of phosphoric acid and which is notnormally an oxidizing agent for vanadium during the catalystpreparation.The acid precursor of the anion may be either organic Acids .such ashydrochloric, .hydroiodic, hydrobromic, acetic, oxalic, malic, citric,formic and mixtures thereof such as a mixture of hydrochloric on oxalicmay be used. Less desirably, sulfuric and hydrofluoric may be employed.Other reducing agents which may be employed, but which have not given asdesirable catalysts' are organic aldehydes such as formaldehyde andacetaldehyde; alcohols such as pentaerythritol, diacetone alcohol anddiethanol amine, and additional reducing agents such as hydroxyl amines,hydrazine, sulphur dioxide and nitric oxide. Nitric acid and similaroxidizing acids which would oxidize the vanadium from a valence of 4 toduring the preparation of the catalyst should be avoided. Vanadiumoxysalts formed from the inorganic acids have given excellent results,and the best results have been obtained using the salt from hydrochloricacid; that is, using vanadium oxychloride.

The phosphorus stabilizer may be added as the element or elements per seor as the compounds thereof such as the oxides, hydroxides, carbonates,phosphates, nitrates, sulfates, and inorganic or organic salts. Examplesof these compounds are arsenic trioxide, ferrous oxide, ferric oxide,cobaltous oxide, aluminum hydroxide, aluminum oxide, zinc chloride,titanium tetrachloride, chromium nitrate, cupric chloride, cadmiumoxide, and the respective oxides of gallium, indium, scandium, yttrium,lanthanum, germanium, lead, bismuth, cerium, praseodymium, neodymium andchromium.

Although the catalysts may be separately formed and used as pellets, itis more economical and practical to deposit this material on a carriersuch as aluminum oxide. Before the carrier is combined with the catalystthe solution of catalyst is preferably concentrated to a solution whichcontains from about 30 to 80% volatiles and better results have beenobtained when there is from about 50 to 70% volatiles by weight. Thecarrier may be added to the catalyst solution or the catalyst solutionmay be poured onto the carrier. Less desirably, the Alundum or othercarrier may be present during the whole course of reactions to providethe desired vanadiumoxygen-phosphorus complex.

The support or carrier for the vanadium phosphate complex, if any,should preferably be inert to both the depositing solution containingthe complex and should be inert under the catalytic oxidationconditions. The support provides not only the required surface for thecatalyst, but gives physical strength and stability to the catalystmaterial. The carrier or support preferably has a low surface area, asusually measured, from about .001 to about 5 square meters per gram. Adesirable form of carrier is one which has a dense non-absorbing centerand a rough enough surface to aid in retaining the catalyst adheredthereto during handling and under reaction conditions. The carrier mayvary in size but preferably is from about 2 /2 mesh to about mesh in theTyler Standard screen size. Alundum particles as large as 4 inch aresatisfactory. Carriers much smaller than 10 to 12 mesh normally cause anundesirable pressure drop in the reactor. Very useful carriers areAlundum and silicon carbide or Carborundum. Any of the Alundums or otherinert alumina carriers of low surface may be used. Likewise, a varietyof silicon carbides may be employed. Silica gel may be used. The amountof the catalyst complex on the carrier may be varied from about 10 toabout 30 weight percent, and more preferably from about 14 to about 24weight percent on an inert carrier such as Alundum. The amount of thecatalyst complex deposited on the carrier should be enough tosubstantially coat the surface of the carrier and this normally isobtained with the ranges set forth above. With more absorbent carriers,larger amounts of material will be required to obtain essentiallycomplete coverage of the carrier. In the case of silicon carbide, about25 percent of catalyst is normally used. Excess catalyst over thatrequired to coat the carrier surface is not necessary and usually willbe lost by mechanical attrition. The final particle size of the catalystparticles which are coated on a carrier will also preferably be about 2/2 to about 10 mesh size. The carriers may be of a variety of shapes,the preferred shape of the carriers is in the shape of cylinders orspheres. Although more economical use of the catalyst on a carrier infixed beds is obtained, the catalyst may be employed in fluid bedsystems. Of course, the particle size of the catalyst used in fluidizedbeds is quite small, varying from about 10 to about 150 microns and insuch systems the catalyst normally will not be provided with a carrierbut will be formed into the desired particle size after drying fromsolution.

Inert diluents such as silica may be present in the catalyst, but thecombined weight of the vanadium, oxygen, phosphorus and phosphorusstabilizer should preferably constitute at least about 50 weight percentof the composition which is coated on the carrier, if any, andpreferably these components constitute at least about 75 weight percentof the composition coated on the carrier, and more preferably at leastabout weight percent. Any remainder other than the vanadium, oxygen,phosphorus and phosphorus stabilizer may be any inert non-catalyticingredient intimately combined with the vanadium, oxygen, phosphorus andthe phos phorus stabilizer as a part of the coating on the carrier.

Prior to use, the catalytic material may be placed in the reactor usedto convert an olefin such as butene-Z to maleic anhydride and may, forexample, be conditioned by passing butene-2 at the rate of 50 grams ofbutene-Z per liter of catalyst per hour in a concentration of 0.7 molepercent butene-2 in air over the catalyst. The temperature may be slowlyraised over a period of 72 hours, to about 350 C. to 550 C. Thereafter,butene-2 in air may be passed over the catalyst, for example, at aconcentration of about 1.2 mole percent butene-2 at the rate of gramsbutene-2 per liter of catalyst per hour and the maleic anhydride productcollected from the gaseous effluent from the reactor. Of course, themaleic anhydride produced may be collected beginning at the start of theconditioning period if desired.

The reaction, involving vapor phase oxidation of olefins to aliphaticdicarboxylic acid anhydrides requires only passing the olefin in lowconcentrations in air over the described catalyst. Once the reaction isbegun, it is self-sustaining because of the exothermic nature thereof.

The flow rate of the gaseous stream through the reactor may be variedwithin rather wide limits, but a preferred range of operations is at therate of about 50 to 300 grams of olefin per liter of catalyst per hourand more preferably about 100 to about 250 grams of olefin per liter ofcatalyst per hour. Residence times of the gas stream will normally beless than about 2 seconds, more preferably less than about one second,and down to a rate, which is easily determined, whereby less efficientoperations are obtained.

A variety of reactors will be found to be useful and multiple tube heatexchanger type reactors are quite satisfactory. The tubes of suchreactors may vary in diameter from about inch to about 3 inches, and thelength may be varied from about 3 to about 10 or more feet. Asmentioned, the oxidation reaction is an exothermic reaction and,therefore, relatively close control of the reaction temperature shouldbe maintained. It is desirable to have the surface of the reactors at arelatively constant temperature and some medium to conduct heat from thereactors is necessary to aid temperature control. Such media may beWoods metal, molten sulfur, mercury, molten lead, and the like, but ithas been found that eutectic salt baths are completely satisfactory. Onesuch salt bath is a sodium nitrate-sodium nitrite-potassium nitrateeutectic constant temperature mixture. An additional method oftemperature control is to use a metal block reactor whereby the metalsurrounding the tube acts as a temperature regulating body. As will berecognized by the man skilled in the art, the heat exchange medium willbe kept at the proper temperature by heat.

exchangers and the like. The reactor or reaction tubes may be iron,stainless steel, carbon-steel, nickel, glass tubes such as Vycor, andthe like. Both carbon-steel and nickel tubes have excellent long lifeunder the conditions of the reactions described herein. Normally, thereactors contain a preheat zone of an inert material such as inchAlundum pellets, inert ceramic balls, nickel balls or chips and thelike, present at about one-half to one-fourth the volume of the activecatalyst present.

The temperature of reaction may be varied within some limits, butnormally the reaction should be conducted at temperatures within arather critical range. The oxidation reaction is exothermic and oncereaction is underway, the main purpose of the salt bath or other mediais to conduct heat away from the walls of the reactor and control thereaction. Better operations are normally obtained when the reactiontemperature employed is no greater than about 100 C. above the salt bathtemperature, under a given set of conditions, at which optimumconversion to maleic anhydride is obtained. The temperature in thereactor, of course, will also depend to some extent upon the size of thereactor and the olefin concentration. Under ,usual operating conditions,in compliance with the preferred procedure of this invention, thetemperature in the center of the reactor, measured by thermocouple, isabout 375 C. to about 550 C. The range of temperature preferablyemployed in the reactor, measured as above, should be from about 400 C.to about 515 C. and the best results are ordinarily obtained attemperatures from about 420 C. to about 500 C. Described another way, interms of salt bath reactors with carbon steel reactor tubes, about 1.0inch in diameter, the salt bath temperature should be controlled betweenabout 350 C. to about 550 C. In any case, the optimum reactiontemperature and salt bath temperature for maximum yield of desireddicarboxylic acid anhydride should be observed. Under normal conditions,the temperature in the reactor ordinarily should not be allowed to goabove about 550 C. for extended lengths of time because of decreasedyields and possible deactivation of the novel catalyst of thisinvention.

The dicarboxylic acid anhydrides may be recovered by a number of wayswell known to those skilled in the art. For example, the recovery may beby direct condensation or by absorption in suitable media, withsubsequent separation and purification of the dicarboxylic acidanhydride.

The pressure on the reactor is not generally critical, and the reactionmay be conducted at atmospheric, superatmospheric or below atmosphericpressure. The exit pressure will be at least slightly higher than theambient pressure to insure a positive flow from the reaction. Thepressure of the inert gases must be sufficiently high to overcome thepressure drop through the reactor.

The exact nature of the final catalyst complexes is not known. However,the vanadium, oxygen, phosphorus and phosphorus stabilizer arechemically bonded together. Even though the exact nature of the catalystis not known, the composition must be maintained within the prescribedlimits. For example, when large amounts of molybdenum, such as one atomof molybdenum, is added to the catalyst containing one atom of vanadiumand 1.1 to 1.6 atoms of phosphorus, the yield of maleic anhydride issignificantly lowered. Not only is the composition of the catalystimportant, but the method of preparation is influential on yield. Withthese catalysts containing the described proportions of vanadium,oxygen, phosphorus and phosphorus stabilizer, it has been discoveredthat materially higher yields of maleic anhydride result when thevanadium has an average valence of plus 4.6 or less, and generally lessthan 4.2 at the time the catalyst complex is combined with the carrier.The average valence of the vanadium is from about plus 2.5 up to 4.6.Vana- T-rade name of Corning Glass Works, Corning. iii, of a high silicaglass composed of about 96 percent silica glass with the remainder beingessentially B208- dium with an average valence within these ranges canbe obtained by adding a reducing acid such as oxalic or hydrochloric toV 0 The blue color of the solution shows that the vanadium has anaverage valence of less than five. The reduction may be made eitherprior to, at the same time or after the addition of the phosphoruscompound to form the complex. The preferred method is to reduce thevanadium prior to the addition of the phosphorus. However, as mentioned,the vanadium should have the average valence of 4.6 or less at the timethe complex is combined with the carrier. If the vanadium has an averagevalence of about 5.0- and is in a concentrated system containing arelatively small amount of volatile liquid, the vanadium-oxygenphosphorus complex will precipitate out as a visible precipitate.Although the exact reasons are not understood, catalysts prepared inthis manner result in relatively lower yields of maleic anhydride. Inthe preferred method of the present invention, the catalyst complex isdeposited directly from solution onto the carrier without going throughan intermediate precipitation step. That is, the catalyst complex isdeposited as a solution onto the carrier. The atomic ratio of oxygen tothe remaining components of the catalyst when the catalyst is being usedto catalyze the oxidation of hydrocarbons to aliphatic dicarboxylicacids is difficult to determine and is probably not constant due to thecompeting reactions of oxidation and reduction taking place during thereaction.

The catalyst of the present invention and the process of using them areuseful for the production of aliphatic dicarboxylic acid anhydrides fromaliphatic hydrocarbons generally. Ethylenically unsaturated hydrocarbonsof from 4 to 6 carbon atoms such as 3-methylbutene-l, isoprene,2,3-dimethyl butadiene are also useful starting materials. The preferredstarting materials are the four carbon hydrocarbons such as butene-l,cis or trans butene- 2 and butadiene-1,3 and mixtures thereof. Usefulfeeds as starting materials may be mixed hydrocarbon streams such asrefinery streams. For example, the feed material may be theolefin-containing hydrocarbon mixture obtained as the product from thedehydrogenation of hydrocarbons. Another source of feed for the presentprocess is from refinery by-products. For example, in the production ofgasoline from higher hydrocarbons by either thermal or catalyticcracking a predominantly C hydrocarbon stream may be produced and maycomprise a mixture of butenes together with butadiene, butane,isobutane, isobutylene and other ingredients in minor amounts. These andother refinery by-products which contain normal ethylenicallyunsaturated hydrocarbons are useful as starting materials. Althoughvarious mixtures of hydrocarbons are useful, the preferred hydrocarbonfeed contains at least 70 weight percent butene-l, butehe-Z and/ orbutadiene-l,3 and mixtures thereof, and more preferably contains atleast percent butene-l, butane-2, and/or butadiene-l,3 and mixturesthereof. Any remainder 'will be aliphatic hydrocarbons.

The gaseous feed stream to the oxidation reactors normally will containair and about 0.5 to about 2.5 mole percent hydrocarbons such as butene.About 1.0 to about 1.5 mole percent of the monoolefin are satisfactoryfor optimum yield of product for the process of this invention.Althou'gh'higher concentrations may be employed, it should be noted thatexplosive hazards may be encountered at concentrations of butene-2 aboveabout 2.0 percent and thus are generally avoided. Concentrations ofbutene-Z less than about one percent, of course, will reduce the totalyields obtained at equivalent flow rates and thus are not normallyeconomically employed. The source of the oxygen may be pure oxygen orsynthetic or natural mixtures of oxygen and inert gases such as nitrogenorhelium may be used. Dry air is entirely satisfactory.

The maleic anhydride product has many well known commercial uses such asa modifier for alkyd resins.

In the following examples a quantity of 6 mm. X 6 mm. Vycor Raschigrings equivalent to about A to of the volume of the catalyst particlesWas loaded into the reactor on top of the catalyst particles (at thereactor inlet) to act as an inert preheat zone. Unless otherwise noted,the amount of catalyst composition coated on the carrier amounts toabout 20 weight percent of the total weight of catalyst and carrier. Inall of the examples, the percent of the phosphorus stabilizer is basedon the total Weight of V and P 0 (or equivalent H PO used. The butenefeed in all of the examples (exclusive of air) contained approximately97 mole percent butene-2 with the remainder being C to C hydrocarbonimpurities. Except where otherwise noted the carrier particles used wereA; x 4; inch cylindrical Alundum which had been washed with hydrochloricacid. The yields of maleic anhydride are calculated on the mole percentof maleic anhydride formed per mole of butene fed. Yield values notedrepresent yields after the yield values had leveled out following theactivation period. The reactor temperatures are the temperatures of thesalt bath. In all of the examples the butene concentration is based onthe combined moles of air and butene.

The examples are only illustrative and are not intended to limit theinvention.

Example 1 A catalyst for oxidation of butene-2 to maleic anhydride wasprepared as follows: 58.4 grams of vanadium pentoxide V 0 was added to600 milliliters of 37% hydrochloric acid. The mixture was refluxedslowly and after the initial reaction the mixture was refluxed for about13 hours. After a blue solution was obtained, showing that the vanadiumhad an average valence of less than plus five, 3 g. of ZnCl was added tothe solution. Thereafter 61.6 g. of P 0 was cautiously added to thesolution and the mixture was again refluxed. The resulting deep bluesolution was evaporated to about 205 milliliters, which solution weighed278.5 grns. To the hot solution was added 480 grams of hydrochloric acidextracted inch x /8 inch cylindrical Alundum pellets. Thevanadium-oxygen-phosphorus complex was deposited on the Alundum and thendried. A free-flowing catalytic material was obtained which had thephosphorus-oxygenvanadium uniformly deposited on the surface of theAlundum. The coated Alundum contained 20 weight percent of the complexbased on the weight of carrier plus catalyst. The catalyst coating hadan atomic ratio of 1.35 atoms of phosphorus to 1.0 atoms of vanadium andcontained 2.5% ZnCl based on the weight of the remaining catalyticcoating.

300 ml. of the catalyst (428 gr.) were loaded into the bottom of a 3foot long, inch I.D. nickel reactor tube surrounded by a salt bath. Ontop of the catalyst was loaded 70 ml. of 6 mm. x 6 mm. Vycor Raschigrings to form a preheat zone. A butene mixture containing 95 to 97 molepercent butene-2 together with the remainder being C to C hydrocarbonimpurities was mixed with air to give a mixture containing about 0.7mole percent butene-2. The mixture of butene and air was fed into thetop of the reactor at a rate of 80 gr. of butene per liter of catalystper hour. At a salt bath temperature of 490 C. the yield of maleicanhydride was 89.2 weight percent based on the weight of butene fed. Themaleic anhydride was recovered by bubbling the gaseous stream throughwater.

Example 2 The general procedure of catalyst preparation and oxidation tomaleic anhydride followed in Example 1 was repeated. The catalyst had anatomic ratio of 1.1 phosphorus, 1.0 vanadium and contained 0.775 weightpercent of Fe O based on the weight of the remaining catalyticingredients. A 3 feet long, inch LD. nickel tube reactor was used andwas loaded with 300 ml. of the catalyst. A 1.0 mol percent butene-2 inair mixture was fed 8 through the reactor at a rate of grams of buteneper liter of catalyst per hour. At a reactor temperature of 455 C. theyield of maleic anhydride was 84.8 weight percent based on the butenefed.

Example 3 Example 2 was repeated with the exception that the atomicratio of the catalyst was 1.5 phosphorus to 1.0 vanadium and the Fc Owas present in a concentration of 1.5 weight percent. At 480 C. reactortemperature, the yield of maleic anhydride was 77.0 weight percent.

Example 4 The general procedure of Example 1 was repeated. The atomicratio of the catalyst was 1.35 phosphorus and 1.0 vanadium and the Fe Owas present at a concentration of 1.25 Weight percent. The reactor usedwas a 1.5 foot long, inch I.D. nickel reactor tube surrounded by atemperature regulating brass block. The concentration or butene was 1.1mol percent and the butene-air mixture was fed at a rate of 108 grams ofbutene per liter of catalyst per hour. At a reactor temperature of 470C. the yield of maleic anhydride was 82.2 weight percent.

Example 5 The general procedure of Example 2 was repeated with thecatalyst having an atomic ratio of 1.35 phosphorus and 1.0 vanadium andwhich contained 1.5 weight percent C0 0 A 3 foot long, 4 inch I.D.nickel reactor tube surrounded by a salt bath was used. 300 ml. of thecatalyst was used in the reactor. The butene-2 feed rate Was 1.3 molpercent and the throughput was 143 grams of butene per liter per hour.At a reactor temperature of 515 C. the yield of maleic anhydride was69.1 weight percent.

Example 6 The general procedure of Example 2 was repeated. The catalysthad an atomic ratio of 1.35 phosphoru and 1.0 vanadium and contained 1.5weight percent NiO [added as Ni(NO The same reactor was used as inExample 5 and the butene concentration of 1.3 mol percent with a feedrate of 148 grams of butene per liter of catalyst per hour. At a reactortemperature of 510 C. the yield of maleic anhydride was 73.5 weightpercent.

Example 7 The general procedure of Example 2 was repeated. The samereactor was used as in Example 5. 300 ml. of a catalyst having an atomicratio of 1.25 phosphorus and 1.0 vanadium and which contained 1.5 weightpercent Al(OH were used. The concentration of butene was 1.3 mol percentand the feed rate was 142 grams of butene per liter of catalyst perhour. At a temperature of 503 C. the yield of maleic anhydride was 73.5weight percent.

Example 8 The general procedure of Example 2 was repeated. The reactorused was a 6 foot long, /1 inch I.D. iron reactor tube surrounded by asalt bath. Into the bottom of the reactor was charged 300 ml. of acatalyst having an atomic ratio of 1.2 phosphorus and 1.0 vanadium andwhich contained 0.7 weight percent Fe O Thereafter 300 ml. of a catalysthaving an atomic ratio of 1.42 phosphorus and 1.0 vanadium and whichcontained 0.7 weight percent R5 0 was charged. Both of these catalystswere coated on 4 to 8 mesh scraggy silicon carbide carrier instead ofthe Alundum used in the previou examples. In both of these catalysts thecatalyst composition was present in a concentration of 25 weight percentof the combined weight of the catalyst and silicon carbide carrier. Theconcentration of butene fed was 1.1 mol percent and the feed rate was 92grams of butene per liter of catalyst per hour. At a reactor temperatureof 490 C. the yield of maleic anhydride Was 89.2 weight percent.

Example 9 The general procedure of Example 2 was repeated. The reactorused was a 1.5 foot long, inch I.D. nickel reactor tube surrounded by abrass block. 150 ml. of a catalyst having an atomic ratio of 1.45phosphorus and 1.0 vanadium and which contained 1.67 weight percent ZnClwas used. The concentration of butene was 0.4 mol percent and the feedrate was 42 grams of butene per liter of catalyst per hour. At atemperature of 455 C. the yield of maleic anhydride was 86.4 weightpercent.

Example 10 The general procedure of Example 2 was repeated. The reactorused was the same as in Example 11. 150 ml. of catalyst having an atomicratio of 1.45 phosphorus and 1.0 vanadium and which contained 2.1 weightpercent of SnCl -2H O was used. The concentration of butene was 0.4 molpercent with a feed rate of 56 grams of butene per liter of catalyst perhour. At a reactor temperature of 450 C., the yield of maleic anhydridewas 84.0 weight percent.

Example 11 The general procedure of Example 2 was repeated. The samereactor was used as in Example 5. 300 ml. of a catalyst having an atomicratio of 1.35 phosphorus and 1.0 vanadium and which contained 4.2 weightpercent TiCl was used. The concentration of butene was 0.9 mol percentand the feed rate was 96 grams of butene per liter of catalyst per hour.At a reactor temperature of 466 C. the yield of maleic anhydride was88.4 weight percent.

Example 12 The same reactor was used as in Example 5. 300 ml. of acatalyst having an atomic ratio of 1.35 phosphorus and 1.0 vanadium andwhich contained 2.5 weight percent ZnCl was used. The concentration ofbutene was 0.7 mol percent and the feed rate was 80 grams of butene perliter of catalyst per hour. At a reactor temperature of 490 C. the yieldof maleic anhydride was 86.4 weight percent.

I claim:

1. In a process for the vapor phase catalytic oxidation of butene toprovide maleic .anhydride, the improvement which comprises effectingsaid catalytic oxidation in the presence of a catalyst complexcomprising vanadium, oxygen, phosphorus and a metal selected from thegroup consisting of the metals of the fourth period of Group VIIIb andmixtures thereof, said catalyst containing from about 1.1 to about 1.6atoms of phosphorus per atom of vanadium and containing from about 0.20to about 2.0 weight percent of the said metals of the fourth period ofGroup VIIIb based on the total weight of vanadium, oxygen andphosphorus; the said catalyst having been deposited onto carrierparticles while the vanadium has an average valence of no more than plus4.6.

2. A process for the production of maleic anhydride which comprisespassing in the vapor phase a mixture of butene and oxygen through areactor packed with a catalyst, said catalyst being avanadium-oxygen-phosphorus complex having chemically bonded therewith aphosphorus stabilizer selected from the group consisting of metals ofthe fourth period of Group VIIIb and mixtures thereof, said catalystcontaining from about one to about two atoms of phosphorus per atom ofvanadium and containing from about 0.05 to about 5.0 weight percent ofthe said phosphorus stabilizer based on the total weight of vanadium,oxygen and phosphorus; the said catalyst having been prepared insolution, by mixing a vanadium oxysalt wherein the salt-forming anion isan anion which is more volatile than the phosphate anion, with the saidphosphorus stabilizer and a member selected from the group consisting ofphosphoric acid and phosphorus pentoxide.

3. In a process for the vapor phase catalytic oxidation of butene toprovide maleic anhydride, the improvement which comprises efiecting saidcatalytic oxidation in the presence of a catalyst complex comprisingvanadium, oxygen, phosphorus and a phosphorus stabilizer selected fromthe group consisting of the metals of the fourth period of Group VIIIband mixtures thereof, said catalyst having from about 1.1 to about 1.6atoms of phosphorus per atom of vanadium and having from about 0.05 toabout 5 .0 weight percent of the said phosphorus stabilizer based on thetotal weight of vanadium, oxygen and phosphorus; the said catalysthaving been deposited on the carrier particles while the vanadium has anaverage valence of no more than plus 4.6.

4. A process according to claim 3 wherein the said catalyst is depositedon the carrier in an amount whereby the said catalyst amounts to about10 to 30 weight percent of the total weight of catalyst plus carrier.

5. A process for the production of maelic anhydride which comprisespassing in the vapor phase a mixture of butene and oxygen through areactor packed with a catalyst, said catalyst comprising avanadium-oxygenphosphorus complex having chemically bonded therewithiron as a phosphorus stabilizer, said catalyst containing from about oneto about two atoms of phosphorus per atom of vanadium and containingfrom about 0.05 to about 5.0 weight percent of the said iron based onthe total weight of vanadium, oxygen and phosphorus, the said catalysthaving been prepared in solution by mixing a vanadium oxysalt whereinthe salt-forming anion is an anion which is more volatile than thephosphate anion, with the said iron and a member selected from the groupconsisting of phosphoric acid and phosphorus pentoxide.

6. A process for the production of maleic anhydride which comprisespassing in the vapor phase a mixture of butene and oxygen through areactor packed with a catalyst, said catalyst comprising avanadium-oxygen-phosphorus complex having chemically bonded therewithcobalt as a phosphorus stabilizer, said catalyst containing from aboutone to about two atoms of phosphorus per atom of vanadium and containingfrom about 0.05 to about 5.0 weight percent of the said cobalt based onthe total weight of vanadium, oxygen and phosphorus, the said catalysthaving been prepared in solution by mixing a vanadium oxysalt whereinthe salt-forming anion is an anion which is more volatile than thephosphate anion, with the said cobalt and a member selected from thegroup consisting of phosphoric acid and phosphorus pentoxide.

7. A process for the production of maleic anhydride which comprisespassing in the vapor phase a mixture of butene and oxygen through areactor packed with a catalyst, said catalyst comprising avanadium-oxygenphosphorus complex having chemically bonded therewithnickel as a phosphorus stabilizer, said catalyst containing from aboutone to about two atoms of phosphorus per atom of vanadium and containingfrom about 0.05 to about 5.0 weight percent of the said nickel based onthe total weight of vanadium, oxygen and phosphorus, the said catalysthaving been prepared in solution by mixing a vanadium oxysalt whereinthe salt-forming anion is an anion which is more volatile than thephosphate anion, with the said nickel and a member selected from thegroup consisting of phosphoric acid and phosphorus pentoxide.

References Cited in the file of this patent UNITED STATES PATENTS2,496,621 Deery Feb. 7, 1950 2,773,838 Reid et al. Dec. 11, 19562,920,049 Romanovsky et a1. Jan. 5, 1960 2,938,874 Rosinski May 31, 19602,959,600 Houben Nov. 8, 1960 2,992,236 Bavley et al. July 11, 1961

1. IN A PROCESS FORTHE VAPOR PHASE CATALYTIC OXIDATION OF BUTENE TOPROVIDE MALEIC ANHYDRIDE, THE IMPROVEMENT WHICH COMPRISES EFFECTING SAIDCATGALYTIC OXIDATION IN THE PRESENCE OF A CATALYST COMPLEX COMPRISINGVANADIUM, OXYGEN, PHOSPHORUS AND A METAL SELECTED FROM THE GROUPCONSISTING OF THE METALS OF THE FOURTH PERIOD OF GROUP VIIIB ANDMIXTURES THEREOF, SAID CATALYST CONTAINING FROM ABOUT 1.1 TO ABOUT 1.6ATOMS OF PHOSPHORUS PER ATOM OF VANADIUM AND CONTAINING FROM ABOUT 0.20TO ABOUT 2.0 WEIGHT PERCENT OF THE SAID METALS OF THE FOURTH PERIOD OFGROUP VIIIB BASED ON THE TOTAL WEIGHT OF VANADIUM, OXYGEN ANDPHOSPHORUS; THE SAID CATALYST HAVING BEEN DEPOSITED ONTO CARRIERPARTICLES WHILE THE VANADIUM HAS AN AVERAGE VALENCE OF NO MORE THAN PLUS4.6.